Spectral sensitization is an extremely important and indispensable technique in the preparation of a light-sensitive material having a high sensitivity and an excellent color reproducibility. A spectral sensitizing dye is inherently adapted to absorb light in a long wavelength range which is not substantially absorbed by a silver halide photographic emulsion to transfer the light energy to the silver halide. Accordingly, the expansion of the light-sensitive wavelength range and increase of the photographic sensitivity by such a spectral sensitizing agent is an essential technology. Thus, attempts have been made to increase the captured amount of light by increasing the amount of spectral sensitizing agent added to the silver halide emulsion. However, if the spectral sensitizing dye is added to the silver halide emulsion in excess of the optimum amount, it may cause a great desensitization. This phenomenon, generally known as dye sensitization, occurs when desensitization takes place in the inherent light-sensitive range of silver halide in which no light is absorbed by the sensitizing dye. A great desensitization deteriorates the total sensitivity despite its spectral sensitizing effect. In other words, a decrease in the dye desensitization accordingly causes an increase in the sensitivity in the range of light absorption by the sensitizing dye (i.e., spectral sensitivity). Thus, improvement in the inhibition of dye desensitization is of central importance in the spectral sensitization process. Furthermore, the dye desensitization becomes remarkable for a sensitizing dye having a light-sensitive range in a longer wavelength range. This is further described in C. E. K. Mees, The Theory of the Photographic Process, Macmillan, 1942, pp. 1067-1069.
In order to increase the sensitivity by decreasing the dye desensitization, various approaches are disclosed in JP-A-47-28916, 49-46738 and 54-118236 (the term "JP-A" as used herein refers to an "unexamined published Japanese patent application") and U.S. Pat. No. 4,011,083 which suggest the use of acetylene-silver complexes, pyrazolone compounds, N-aminoimmonium compounds, N-imine compounds or azaindene-silver complexes. Other approaches are disclosed in JP-B-45-22189 (British Pat. No. 1,211,735) (the term "JP-B" as used herein refers to an "examined Japanese patent publication"), JP-A-46-550 (U.S. Pat. No. 3,615,613), JP-A-54-18726 (U.S. Pat. No. 4,212,672), JP-A-52-4822 (U.S. Pat. No. 4,046,572) and JP-A-52-151026 and U.S. Pat. No. 2,945,762 which suggest the combined use of a bisaminostilbene compound substituted by a pyrimidine derivative or triazine derivative. These approaches are now deemed to be the most effective method. Further approaches are disclosed in U.S. Pat. Nos. 3,695,888 and 3,809,561 and British Pat. Nos. 1,064,193 and 1,255,084 which suggest the combined use of a bisaminostilbene compound substituted by a triazine derivative and ascorbic acid compound. In accordance with these patents, this process improves the inhibition of the dye desensitization, providing a high sensitivity. However, these approaches have proved inadequate in their effects. Thus, it has been desired to further improve the dye sensitivity.
As a result of intensive studies, the inventors found that the combined use of an ascorbic acid compound and a bisaminostilbene compound substituted by a pyrimidine derivative inhibits desensitization caused by a red-sensitive sensitizing dye, thus providing a remarkably high increase in red dye sensitivity.